1. Field of the Invention
The invention relates to a current amplifier having an input terminal and an output terminal intended to receive and supply an input current and an output current, respectively, comprising a first and a second transistor each having a bias terminal, a transfer terminal and a reference terminal, the reference terminals of the first and second transistors being interconnected via a first resistor.
2. Description of the Related Art
Among multiple possible applications, such an amplifier is often used for forming an input stage, called preamplifier, within a low-noise amplifier intended to amplify a signal coming from a frequency converter in an apparatus for receiving electromagnetic signals. In the current low-noise amplifiers, the preamplifier is constituted by the first and second transistors arranged in a differential pair configuration, in which a first resistor which connects their reference terminals, then forms a degeneration resistor. If, for example, the first and second transistors are bipolar transistors, their bias terminals, transfer terminals and reference terminals will be constituted by their bases, collectors and emitters, respectively. The advantage of the known preamplifier is that its gain, defined as the ratio between the value of the AC component of the output current and that of the input current, and, consequently, the gain of the overall low-noise amplifier, can be easily controlled by adjusting the value of the first resistor. However, the known preamplifier has a high input impedance because its input terminal is constituted by the bias terminal of one of the first or second transistors. In conformity with Ohm's law, this strong input impedance induces the necessity for the frequency converter to generate a signal having a considerable voltage swing, and thus involves a strong energy consumption, which is a considerable drawback in an era in which one of the major aims of research and development in the manufacture of integrated circuits is precisely the reduction of the energy consumption in such circuits. Moreover, a high input impedance of the preamplifier induces constraints in the dimensioning of passive elements, such as inductances or capacitances, included in the frequency converter. Indeed, the nominal values of these elements should be lower as the input impedance of the amplifier is higher. The real values of these passive elements will inevitably exhibit differences with respect to their nominal values, due to imperfections related to the employed manufacturing process, which differences will influence the behavior of the frequency converter to a larger extent as the nominal values are lower. Thus, of a too high input impedance of the preamplifier may introduce unpredictabilities in the functioning of the frequency converter, which is to be prohibited.